This invention relates to extrusion dies for an apparatus producing seamless plastic tubing, particularly, pipes with a smooth inner surface and an annularly ribbed or finned outer surface defined by alternating ribs and grooves.
Known dies of such a type (for example, U.S. Pat. Nos. 3,891,007, 3,998,579 and 4,365,948) include an extrusion head with a nozzle within which a hollow mandrel is placed and defining an annular extrusion orifice forming a parison of the tubing.
The major drawback of these dies is that their mandrel extends substantially beyond the nozzle and forms a large cavity wherein the material is cooled, less viscous materials (such as polivinyl chloride) being therefore not employable. Also, the known dies do not allow to vary the wall thickness of the tubing.